Chemosensory responses are critical components in the control of several essential behaviors of insects that are vectors for pathogens responsible for many important human diseases. In particular, olfaction plays a major role in host seeking and oviposition selection behaviors of blood-feeding female mosquitoes and, as such, constitutes a critical component of the mosquito's ability to transmit diseases such as malaria, dengue, yellow fever and West Nile virus encephalitis. Within this context, and together with our colleagues, we have undertaken a molecular and cellular examination of several elements of the olfactory signal transduction cascade in the principal African malaria vector mosquito Anopheles gambiae sensu stricto and the arbovirus vector Aedes aegypti. An increased understanding of olfactory mechanisms and their underlying chemical cues in these systems may provide insight into the processes of insect behavioral responses in general and disease transmission by vectors in particular and would likely be instrumental in the development of novel mosquito control strategies. We have made significant progress in the characterization of several aspects of olfactory process in both mosquito systems leading to, among other things, the identification and initial characterizations of a family of odorant receptor proteins in An. gambiae (AgORs) and Ae. aegypti (AaORs) that lie at the heart of the olfactory signaling pathway. Building on those advances, this proposal for competitive renewal focuses on extending several elements of our ongoing program including characterization of: (1) structure/function relationships between AgORs and AaORs, (2) the role of AgORs and cognate odorant binding proteins (AgOBPs) in the context of larval olfaction and odor coding in vivo and (3) the molecular events underlying oviposition site selection by gravid adult females.